This invention relates to the recovery of data from rotating data storage disks such as, typically, compact disks (CDs) used as read-only memories (ROMs) in computer systems. More specifically, the invention concerns how to determine the optimum speed of rotation for reading each CD-ROM or like data storage disk according to the particular eccentricity of that disk.
Although originally developed for consumer audio reproduction in substitution for analog phonograph records, CDs have now found wide-spread commercial acceptance as compact, high-speed, large-capacity ROMs for computers. The only difference of CD-ROMs from audio CDs is that CD-ROMS are driven at speeds several times as high as the standard audio CD speed (1.2-1.4 meters per second).
There have been problems left unsolved in use of CD-ROMs with high-speed CD-ROM drives in computer systems. CD-ROMs are presently being made under the same design specifications and manufacturing standards as low-speed audio CDs. Inevitably, therefore, a substantial percentage of CD-ROMs on the market are not of truly satisfactory quality for use as high-speed computer data storage media. Some have their spindle holes cut eccentrically; others have their multiturn spiral tracks not centered at the geometric center of the disk.
Such eccentric disks, particularly eccentric hole disks, are easy to vibrate when driven at a speed as high as, say, twenty times the standard audio CD speed. The disk vibration can be resolved into horizontal and vertical components, it being understood that the disk is laid horizontally. The horizontal component can overload the tracking servo of the CD-ROM drive whereby the radial position of the beam spot on the disk is maintained in alignment with the spiral data track. The overloading of the tracking servo has resulted in the overheating of the voice-coil actuator for tracking control, in the degradation of its performance and, in the worst case, in the burning of the actuator coil.
The vertical component of disk vibration, on the other hand, can cause the overloading of the beam focusing servo of the CD-ROM drive, again resulting in the overheating of the focusing actuator of voice-coil construction, in the degradation of its performance and, in the worst case, in the burning of the focusing actuator coil. The malfunctioning, not to mention total destruction, of the tracking or the focusing servo must be averted by any means because of ensuing read errors and retries, which is a waste of time running counter to the objective of high speed data recovery for which CD-ROMs are intended.
Disk vibration can give rise to additional inconveniences. A vibrating disk may cause vibration of the complete CD-ROM drive and, in consequence, of the hard disk drive (HDD) in particular which is currently being preferentially built into the computer system along with the CD-ROM drive. The vibration of the HDD is just as undesirable, causing both read and write errors, to the detriment of the reliability of the complete computer system. Even if not so excessive as to cause read and write errors, the vibration of the CD-ROM drive certainly gives a shoddy image of the product to the user or to prospective buyers.
The reader might think that all such troubles and inconveniences would not manifest themselves if only high quality CD-ROMs, altogether free from the physical defects pointed out earlier, were used. CD-ROM drive manufacturers cannot, and should not, expect their products to be used only with such high quality disks, since in the current state of affairs the existence of poor quality CD-ROMs must be more or less taken for granted.
Fujimoto et. al. U.S. patent application Ser. No. 08/890,924, filed Jul. 10, 1997, teaches to check the eccentricity of each CD-ROM loaded into the disk drive and to read the disk at a speed matching its particular eccentricity. The maximum possible reading speed according to this prior related application is assumed to be eight times the standard audio CD speed. More recently, however, CD-ROMs have begun to be read at as high as twenty or even thirty times the standard audio CD speed. The higher the speed of rotation, the more will the disk vibrate if it is eccentric. A more accurate determination of the amount, and "nature", as will be later explained, of the eccentricity of each disk has proved necessary for reading only high quality disks at such ultrahigh speeds, and other disks at less speeds matching their particular eccentricities.